DESCRIPTION: Cytochrome bc/bf complexes play a critical role in energy metabolism of cells by coupling redox reactions to the translocation of protons across a biological membrane for the generation of a proton motive force that is used to synthesize ATP. The main working model for the operation of these complexes is the Q-cycle which involves oxidation and reduction of quinone/quinol at two independent sites on the complex. This proposal has focused on structural and functional aspects of the cytochrome b6f complex in oxygenic photosynthetic membranes. They will define the role of individual components in the reactions of the complex as well as in proposed roles related to the assembly/stabilization of the complex. They will consider structural features of the holocomplex as well as study an important regulatory activity that has recently been identified. In terms of the complex in situ, they will measure electron transfer reactions using a flash-kinetic approach, in order to evaluate proposed mechanisms. They will consider in some detail the molecular nature of electron transfer proteins and other non-prosthetic binding proteins in the complex, using the green alga, Chlamydomonas, in a molecular genetic approach. This will involve site-directed mutagenesis for modification of specific amino acids in these proteins to define their functions, taking advantage, in one case, of the structural information that has recently been provided for cytochrome f. Finally, they will attempt to define the role of the cytochrome b6f complex in regulating a kinase that phosphorylates membrane proteins. These studies will provide important findings relating to the function of energy transducing membrane protein complexes.